Liquefaction of natural gas at supercritical pressure employing a single refrigeration cycle



Dec. 3, 1968 KNIEL 3,413,817

LIQUEFACTION OF NATURAL GAS AT SUPERCRITICAL PRESSURE EMPLOYING A SINGLE REFRIGERATION CYCLE] Filed July 6, 1967 Fuel Gas INVENTOR.

Ludwig Kniel ATTORNEYS United States Patent 0 3,413,817 LIQUEFACTKON OF NATURAL GAS AT SUPER- ClRITiCAL PRESSURE EMPLUYENG A SINGLE REFRIGERATION CYCLE Ludwig Kniel, Scarsdale, N.Y., assignor to The Lnmmus Company, New York, N.Y., a corporation of Delaware Continuation-impart of application Ser. No. 437,621 Feb. 18, 1965, which is a continuation-in-part or application Ser. No. 358,789, Apr. 16, 1964. This application July 6, 1967, Ser. No. 651,587

9 Claims. (Cl. 6223) ABSTRACT OF THE DISCLOSURE A process for liquefying natural gas by compressing the gas to a pressure above the critical pressure thereof, cooling the gas by indirect heat transfer with a single refrigerant to a temperature slightly above the normal boiling point of the refrigerant and expanding the cooled gas to effect liquefaction thereof.

The present application constitutes a continuation-inpart of application Ser. No. 437,621, filed Feb. 18, 1965, now Patent No. 3,342,037 which is a continuation-in-part of US. patent application Ser. No. 358,789, filed Apr. 10, 1964, now abandoned.

This invention relates to a process for the liquefaction of a gas, and more particularly relates to a process and apparatus for the liquefaction of natural gas primarily comprised of methane, and including heavier hydrocarbons such as ethane, propane, butane and the like.

There are many reasons for reducing natural gas to a liquefied state. One of the main reasons for liquefying natural gas is the resultant reduction of the volume of the gas to about ,6 of the volume of natural gas in the gaseous state. Such a reduction in volume permits the storage and transportation of liquefied natural gas in containers of more economical and practical design. Additionally, it is desirable to maintain a supply of gas through peak demand periods whereby such periods can be met by liquefied gas held in storage. Another important reason is the transportation of liquefied natural gas from a source of plentiful supply to a distant market where the source and supply may not be eflicaciously joined by pipe lines and transportation in the gaseous state would be uneconomical.

The object of this invention is to provide an apparatus and method for the economical and efiicient liquefaction of a gas, particularly natural gas, for storage and transportation.

Another object of this invention is to provide a process for the liquefaction of natural gas wherein the natural gas is compressed to a pressure above the supercritical pressure, and passed in heat exchange relationship through a refrigeration system, to remove, at relatively high temperatures, the heat as sensible heat, which would be, at lower pressures, latent heat at lower temperatures.

A further object of my invention is to provide a process for liquefying natural gas wherein the gas is compressed to a pressure above the supercritical pressures, cooled in a refrigeration system having a single refrigerant to conveniently remove at relatively high temperatures the sensible heat of the natural gas, which would at lower pressures result as latent heat at lower temperature, and thereafter expanding the thus cooled natural gas in a series of stages to produce liquefied natural gas at atmospheric pressure and at a temperature of about 258 F.

A still further object of my invention is to immediately compress the vapors removed from such expansion stages and combine such vapors with the feed introduced into the initial compressor wherein the pressure of the gas is 3,4135 l7 Patented Dec. 3, 1968 raised to a pressure of about 1.3 times the critical pressure thereof.

Still another object of my invention is to provide a method and apparatus for the liquefaction of natural gas wherein the quantity of nitrogen in the process is maintained at a low level, and the liquefied product is substantially free of nitrogen.

Other objects and fuller understanding of my invention may be had by referring to the following description taken in conjunction with the accompanying drawing, in which,

The drawing is a simplified, schematic flow diagram of an embodiment of the invention.

The natural gas to be treated in accordance with my invention will be a gas from which a part of the moisture and acid gases, such as carbon dioxide, hydrogen sulfide and the like have been removed in a manner familiar to those skilled in the art. In one embodiment of the invention, the natural gas after such treatment, and a recycle gas stream as hereinafter described, is introduced into a feed gas compressor wherein the pressure of the combined stream is increased from a pressure of about 600 p.s.i.g. to a pressure above the critical pressure of the gas, generally a pressure of about 1.3 times the critical pressure depending on the composition of the gas. The compressed gas is thereafter passed through a cooler and a first heat exchanger of a refrigeration system to remove any condensible heavier hydrocarbons. The cooled gas is then passed through the remaining heat exchangers of such refrigeration system to refrigerate the gas to a temperature slightly above the normal boiling point of the refrigerant. In the case of ethane, as the refrigerant, the normal boiling temperature would be about l25 F. and a natural gas primarily comprised of methane would be cooled to about or slightly above the critical temperature. The refrigerant preferred is ethane since it may be supplied by the natural gas. Of course, other refrigerants can be used provided they may be condensed with a cooling medium at a temperature of about 60 F. or less. Only one refrigerant is utilized in the refrigeration system. While a portion of the water has been removed in the gas prior to compression, additional moisture will be contained in the natural gas after cooling, and accordingly, is conveniently removed between the first and second heat exchangers of such refrigeration system.

The natural gas at a temperature slightly above the normal boiling point of the refrigerant is thereafter expanded into a first flash drum wherein a substantial portion of the gas is liquefied during the expansion. The gaseous overhead from the first flash drum, including nitrogen, is withdrawn and a portion thereof withdrawn from the process as fuel gas, after passing such portion through a heat exchanger to recuperate the cold therefrom. The liquefied natural gas from the first flash drum is thereafter successively expanded over a series of flash drums wherein the pressure of the liquefied natural gas is eventually reduced to atmospheric pressure. The liquefied natural gas is withdrawn from the last flash drum and passed to storage and transportation.

The flashed gases from each of the flash drums, except the last flash drum, are introduced into intermediate stages of a methane recycle compressor and thereafter combined with the natural gas feed. Such combined gas stream is passed to the feed gas compressor wherein the pressure of the gas is raised to a pressure within the aforementioned pressure range. There is a slight reduction in pressure of the liquefied natural gas from the last flash drum into the storage tank, and consequently a portion of the liquefied natural gas will vaporize in the storage tank. Such vaporized portion of the natural gas is also passed to a methane recycle compressor after passing through a centrifugal booster compressor.

Referring to the figure, which is a schematic flow diagram of an embodiment of the invention, the following describes this embodiment applied to the liquefaction of natural gas. It is understood, however, that the invention is also applicable to the liquefaction of other gases containing hydrocarbons, such as refinery gases and the like.

Lean natural gas, primarily comprised of methane, in line 10, is combined with a recycled gaseous stream, as more fully hereinafter described, in line 11, and passed through line 12 to a feed gas compressor 13. The natural gas is compressed from a pressure of about 600 p.s.i.g. to a pressure of about 1.3 times the critical pressure of the gas stream. The compressed gas is passed through line 14 to cooler 15 and thence through line 17 to a heat exchanger 18 of a refrigeration system including heat exchangers 18, 19, 20 and 21. The cooled gas is then passed through line 22 into a separator 23. In separator 23, any heavier hydrocarbons which may condense during passage through cooler 15 and heat exchanger 18 are withdrawn from separator 23 through line 24 and the compressed natural gas withdrawn through line 25 and passed to dryers 26 and 27 to remove residual amounts of moisture in the gas. The dryers 26 and 27 are operated in an alternate manner which is well known to those skilled in the art.

The dried gas in line 28 is split and a portion in line 28a passed through the remaining heat exchangers 19, 20 and 21 of the refrigeration system wherein the gas is cooled by a refrigerant which is expanded into the heat exchangers of the refrigeration system.

The refrigeration system utilizes a single refrigerant as will be more fully hereinafter described. An important aspect of my invention is that the natural gas passing through the refrigeration system is cooled, without liquefaction, to a temperature slightly above the normal boiling point of the refrigerant providing the cooling requirements for the refrigeration system. The gas is readily cooled at such high pressures by using a single refrigerant whereby the heat is conveniently removed at relatively higher temperatures as sensible heat, which would, at lower pressures, show up as latent heat at a lower temperature.

The natural gas in line 28a together with another gaseous stream as more fully hereinafter described, is expanded across valve 29 to a lower pressure, such that a substantial portion of the natural gas is liquefied during such expansion and is thereupon introduced into flash drum 30. The liquefied gas in flash drum 30 is withdrawn through line 32, expanded across valve 33 and introduced into a second flash drum 34 whereby the temperature of the liquefied gas is further reduced. A geaseous phase in flash drum 34 is withdrawn through line 35 while the further cooled liquefied gas is withdrawn through line 36. The liquefied gas in line 36 is thereafter further expanded across valve 37 and introduced into flash drum 38 wherein the liquefied gas is still further cooled. The expansion of the gas in line 28a and the liquefied gas in lines 32 and 36 are to lower pressures, such that the pressure of the liquefied gas in the last flash drum 38 is substantially at atmospheric pressure.

A gaseous phase is withdrawn from flash drum 38 through line 39 While the liquefied natural gas at about atmospheric pressure is withdrawn through line 40. The liquefied natural gas in line 40 is passed by pump 41 through line 42 and introduced into a storage tank, generally indicated as 43, maintained at atmoshperic pressure. Since the storage tank 43 is maintained at about atmospheric pressure, the temperature of the liquefied gas is about 257 F. If there is a small reduction in the pressure of the liquefied gas during passage to the storage tank 43 from flash drum 38, additional cooling of the liquefied natural gas occurs thereby vaporizing a portion of the liquefied gas. The vapors formed during such additional expansion are withdrawn from the storage tank 43 through line 44, and compressed in a centrifugal compressor 45.

As a distinct feature of my invention, I contemplate taking the flashed gases from the flash drums and the storage tank and compressing such vapors in a recycle compressor 46 and combining such compressed stream with the natural gas feed to the plant. The gaseous phases in lines 31a and 35 are passed to an intermediate stage of, the methane compressor 46. The gaseous phase in line 39 is combined with the compressed gas stream in line 47 and passed via line 48 to an initial stage of the methane compressor 46.

Loading the Warm tanks of a vessel with the liquefied natural gas from storage tank 43 through line 49 will effect vaporization of a small portion of the liquefied natural gas which is then returned to the plant through line 50, compressed in dockside centrifugal compressor 51 and is combined with the gas in line 43 through line 52 for compression in compressor 46. The now compressed gas is withdrawn from compressor 49 through line 11 to be combined with the natural gas feed to the plant in line 10.

Most natural gases contain nitrogen as a contaminant. It is desirable in the course of liquefaction to remove as much as possible of the nitrogen from the gas since nitrogen in the liquefied natural gas reduces the value thereof, particularly when carrying the gas in bulk transportation. In accordance with equilibrium conditions in flash drum 30, the gaseous phase in line 31 contains a higher percentage of the nitrogen than the liquid phase in line 32, introduced into the process. A portion of such gaseous phase in line 31 is withdrawn through line 60 and is utilized as a fuel gas in compressor drives and other equipment of the plant.

Since the gas in line 60 is at a relatively low temperature, it is desirable to recover the cold potential thereof prior to using such gas as a fuel gas. To recover such potential, a portion of the compressed gas in line 28 is passed through line 61 under the control of valve 62 to heat exchangers 63 and passed in heat exchange relation with the gas in line 60. Once equilibrium has been reached, the amount of nitrogen introduced into the process with the natural gas feed will equal the amount of nitrogen in the fuel gas in line 60, and the liquefied natural gas in storage tank 43. That portion of the compressed gas in line 61 is withdrawn from heat exchanger 63 through line 64 under control valve 65 and combined with the gas leaving heat exchanger 21 prior to expansion through valve 29. The gas .in line 60 after passage through heat exchangers 63 is withdrawn through line 66.

As a distinct advantage of my system as compared to prior processes, I am able to utilize one refrigerant, such as ethane, to provide the cooling requirements for refrigeration systems. Liquefied ethane in line 67 is expanded stagewise into heat exchangers 18, 19, 20 and 21 to remove the sensible heat of the compressed gas in line 28. The thus expanded ethane is withdrawn from heat exchangers 18, 19, 20 and 21 through lines 68, 69, 70 and 71, respectively, and is passed to an ethane compressor 72. The gaseous ethane in lines 68, 69 and 70 are introduced into intermediate stages of the compressor 72, whereas the gaseous ethane in line 71 is introduced into a first stage thereof. The compresesd ethane is passed through line 73 to heat exchangers 74 wherein the ethane is cooled with a cooling medium having a temperature less than about 60 F., whereby the compressed ethane is condensed into the liquid phase. The liquefied ethane is passed through line 75 to ethane receiver 76. The fuel gas in line 66 may be passed through ethane receiver 76 to further recover the cold potential of the fuel gas stream and is subsequently passed through line 77 to the points of use (not shown).

The following is an example of my invention pertaining to a specific natural gas wherein the quantity flow in pounds actually represents the ratio of flow in pounds per pound of LNG withdrawn from the process. Such natural gas has the following analysis:

Methane 99.5 Ethane .l Nitrogen .4

With reference to the figure, 1.119 pounds of natural gas at a temperature of 60 F. in line after being treated to remove acid gases and moisture is combined with 1.2693 pounds of recycled gas at a temperature of F. in line 11. The combined gas stream at a temperature of 16 F. and at a pressure of 600 p.s.i.a. is compressed to 1400 p.s.i.a. in compressor 15 and cooled to a temperature of 55 F. in heat exchanger 15 during passage against a cooling medium at a temperature of 50 F. The compressed gas is further cooled in heat exchanger 18 to a tempertaure of 5 F. and passed through dryers 26 and 27 wherein residual moisture is removed.

The compressed gas is thereafter serially passed through heat exchangers 19, and 21 and cooled to a temperature of 115 F. The gas withdrawn from heat exchanger 21 is at a pressure of 1360 p.s.i.a. as a result of a pressure drop of 40 p.s.i.a. through the aforementioned units. The gas is then expanded across expansion valve 29 into flash drum 30 to a pressure of 180 p.s.i.a. with a resulting decrease in temperature to 184 F. 1.4115 pounds of liquefied gas is withdrawn from flash drum 30 and expanded across valve 33 to a pressure of 55 p.s.i.a. and introduced into flash drum 34. 1.1458 pounds of liquefied gas now at a temperature of 226 F. is then withdrawn from flash drum 34 and expanded across valve 37 to a pressure of 20 p.s.i.a. and introduced into flash drum 38 wherein the liquefied gas is further cooled to a temperature of 252 F. The liquefied gas in flash drum 38 is withdrawn through line 40 by pump 41 and passed to storage tank 43 maintained at 14.7 p.s.i.a. The additional slight expansion from flash drum 38 and storage tank 43 effects further cooling of the liquefied gas to a temperature of 257 F.

0.8578, 0.2657 and 0.1176 pound of a gaseous phase in lines 31a, and 39 are passed together with 0.0282 pound of gas withdrawn from storage tank 43, to methane recycle compressor 46 and compressed to a pressure of 600 p.s.i.a. and subsequently combined with the feed natural gas. 0.119 pound of flashed gas from flash drum 30 at a temperature of 184 F. is passed through heat exchanger 63 and ethane receiver 76 and withdrawn from the process as fuel gas in line 77. It is understood to those skilled in the art that the above example is with respect to the natural gas of the aforementioned composition and that the temperatures and quantities will vary with the composition of the natural gas.

Gaseous ethane at a temperature of 0 Pl, F., 80 F. and 120" F. is withdrawn from heat exchangers 18, 19, 20 and 21 through lines 68, 69, 70 and 71, respectively, and compressed to 560 p.s.i.a. in compressor 72 and cooled in heat exchanger 74 to a temperature of 70 F. in heat exchange relation with a cooling medium having a temperature of F.

The process of this invention is particularly effective for liquefying a natural gas primarily comprised of methane, i.e., greater than 98% methane, preferably greater than 99% methane. By maintaining pressures above the supercritical pressure of the gaseous stream throughout the cooling cycle, it is possible to use a single refrigerant to remove sensible heat at elevated temperatures which would, at lower pressures, result as latent heat at lower temperatures.

While I have shown and described a preferred embodiment of my invention, I am aware that variations may he made thereto, and, therefore, desire a broad interpretation of my invention within the scope of the disclosure herein and the following claims.

What is claimed:

1. A method of liquefying a hydrocarbon gas comprismg:

(a) compressing said gas to a pressure above the critical pressure thereof;

(b) passing said compressed gas through a heat exchange zone, said gas being cooled by indirect heat transfer in said zone by a single expansion-compression refrigerant cycle to a temperature slightly above the normal boiling point of the refrigerant employed in the cycle, said gas being above the critical pressure during the passage through said heat exchange zone; and

(c) expanding said gas which is at the temperature attained in step (b) to effect cooling and liquefaction thereof.

2. The process of claim 1 wherein said hydrocarbon gas is natural gas.

3. The process of claim 2 wherein said refrigerant is ethane.

4. The process of claim 3 wherein said expansion is effected in a plurality of stages to reduce the pressure of the gas to about atmospheric pressure, said liquefaction occuring in the first stage.

5. The process of claim 4 wherein vapor is separated from the liquefied gas in each expansion stage and said vapor is recycled to step (a).

6. The process of claim 5 wherein a portion of the vapor separated in said first expansion stage is recovered as a fuel gas.

7. The process of claim 6 wherein the cooling in said heat exchange zone is effected in a plurality of stages, with the refrigerant for each stage flowing through a common condensing zone.

8. The process of claim 7 wherein the liquefied gas at atmospheric pressure is passed to a storage tank and the vapor from said storage tank is recycled to step (a).

9. The process of claim 8 wherein said natural gas is primarily comprised of methane.

References Cited UNITED STATES PATENTS 2,901,326 8/1959 Kurata et al.

2,960,837 ll/l960 Swenson 6240 XR 3,092,976 6/1963 Tafreshi 6240 XR 3,160,489 12/1964 Brocoif et al. 6223 XR 3,203,191 8/1965 French 6223 XR 3,257,813 6/1961 Tafreshi 6223 3,271,965 9/1966 Maher et al. 6223 3,292,381 12/1966 Bludworth 6227 XR 3,315,477 4/1967 Carr 6240 XR 2,973,834 3/ 1961 Cicalese.

OTHER REFERENCES Mollier chart, p. 40, Applied Hydrocarbon Thermodynamics, vol. 1, Edmister, published 1961, Gulf Publishing (10., Houston, Texas.

NORMAN YUDKOFF, Primary Examiner.

V. W. PRETKA, Assistant Examiner. 

